Conventional decorative light string has been used for a long time which includes an electrical wire connecting with a plurality of light bulbs. The known bulbs are almost filament-bulbs (1), as shown in FIG. 1. These bulbs will be very hot after being used for a period of time. Therefore, the heat of the filament-bulbs is possible to burn something around them, that might hurt a user who carelessly contacts the bulbs or cause a terrible accident of fire.